This invention employs an ingredient known to improve waterfastness in printing of the jet ink to which it is added, that ingredient being either at a low molecular weight or in combination with a low molecular weight amine. That essential ingredient is hydroxyalkylated polyethyleneimine, preferably hydroxyethylated polyethyleneimine (HEPEI).
No novelty is claimed for the use of hydroxyalkylated polyethyleneimine itself in such inks, and U.S. Pat. Nos. 4,197,135 to Bailey; 4,789,400 to Solodar et al; and 5,017,644 to Fuller et al (Example XIII) disclose such use. The results may not be entirely satisfactory since the hydroxyethylation tends to reduce the effectiveness as a waterfastness aid.
This invention is directed to the elimination of solid whiskers which form spontaneously during inactivity of the ink. No prior art suggestion is known to employ a polyethyleneimine for such purpose. Indeed, no prior art expressly directed to solving the problem of such solid formation is known, although certain ink formulas with pH sensitive dyes have less tendency to solid formation. The common factor in such formulas appears to be that they have at least two dissimilar dyes present in major proportions.
Although this invention is not considered dependent on any factor of the ink formula other than the dye being base soluble, some details of a commercially suitable formula are given. Illustrative prior teachings of various ingredients of that formula are the following: U.S. Pat. Nos. 5,098,476 to Baker illustrating use of 2-pyrrolidone as a co-solvent; 5,062,892 to Halko disclosing buffers such as used in the formulation; 5,049,188 to Takimoto et al and 4,973,992 to Owatari mentioning amines as additives to jet inks.